1. Field of the Invention
The present invention generally relates to hearing aids. The invention, more specifically, relates to a hearing aid with a controllable directional characteristic. The invention, still more specifically, relates to a method for controlling the directionality of the sound receiving characteristic for minimizing noise and to a signal processing apparatus for carrying out the method.
2. The Prior Art
In hearing aids, acoustic signal-to-noise ratio can be significantly improved by e.g. using of dedicated directional microphones or equivalently by a pair of omni-directional microphones followed by a delay and subtracting procedure to employ a directional sound receiving characteristic. Hearing aids with more than two microphones have also been developed in the pursuit of highly selective directionality.
Hearing aids having a directional sound receiving characteristic are useful to improve speech perception in noisy environments, where human speech may be received simultaneously from different directions, as is the case e.g. in the noise environment frequently referred to as the so-called cocktail party noise.
With a directional sound receiving characteristic, e.g., in the shape of a cardoid or super-cardoid characteristic, the speech perception in a hearing aid is improved by reduced perception of sound coming from the back and the sides of the user while maintaining the level of sound coming from the area in front of the user.
On the other hand, in environments with only a low noise level or no significant speech signal, the hearing aid user will normally prefer an omni-directional or spherical sound receiving characteristic offering the same perception of sound irrespectively of the direction, from which it arrives.
To further improve the signal-to-noise ratio, hearing aids with adaptive directional functionality have been introduced with the aim to place significant damping in the direction of the dominant noise source.
WO 01/01731-A1 discloses a method for controlling the directionality of a sound receiving characteristic of a hearing aid. The hearing aid comprises spaced apart microphones, wherein the sound receiving characteristic may change between an omni-directional characteristic and a directional characteristic. In this hearing aid, an adjustable time or phase delay may be imposed. The directional characteristic may be created by adjusting the delay of a delay device to be the same as the acoustic delay between the back microphone and the front microphone. With this delay, signals that are first received at the back microphone and are later received at the front microphone, are suppressed in an adding circuit, where the delayed signal of the back microphone is subtracted from the output signal of the front microphone. The hearing aid may exercise a smooth change-over between an omni-directional characteristic and a directional characteristic, substantially without changing the phase relationship or time delay and the amplitude characteristic of the signal.
Both the fixed and the adaptive directional functions however suffer from a reduced signal-to-noise ratio because of lack of low frequency sensitivity for acoustic signals, since one consequence of adding a signal (from the front microphone) with its delayed and inverted replicate (from the back microphone) to achieve a directional advantage is that the sensitivity of the microphone at low frequencies is reduced also for sounds presented directly in front of the listener. For a given delay and distance between microphones, the low frequency sensitivity rolls off at a rate of 6 dB per octave. This loss of sensitivity in the low frequencies can reduce the overall loudness of sounds, and may effect speech perception and sound quality (see Kuk, F.; Baekgaard, L.; Ludvigsen, C.: Design considerations in directional microphones; in The Hearing Review, September 2002, vol. 7, No. 9, pages 68, 70-73).
To compensate the reduced sensitivity at low frequencies, one could consider a frequency dependent amplification of the microphone signals. However, a frequency dependent amplification will not effect the signal-to-noise ratio but will as a consequence raise the microphone noise by the same amount.
It is therefore an objective of hearing aids with adaptive directional functionality to be able to change from an omni-directional characteristic in quiet situations to a full directional characteristic in noisy environments. Present adaptive systems distinguish between desired signals and undesired signals by the assumption that desired signals, e.g. speech signals, are those coming from the frontal direction of the user of the system, e.g. a hearing aid, whereas undesired signals, e.g. noise signals, are those coming from any other direction. According to this assumption, the signal-to-noise ratio is improved by changing the sound receiving characteristic from an omni-directional mode to a full directional mode since the improvement in signal-to-noise ratio (SNR) is correlated to the directivity index (DI) of a directional microphone.
Present adaptive systems like the directional controller disclosed in WO 02/085066-A1 adjusts the directional characteristic by minimizing the output signal of the system. Since signals coming from the frontal direction are not affected by changing the directional characteristic of the system, a minimization of the output signal results in damping of—and an improvement of the signal-to-noise-ratio. However, such a signal-to-noise-ratio optimization applies only if the desired signals are coming from the frontal direction and noise signals are coming from another direction.
In a situation when a single person is speaking from one side of the user of the adaptive system, the speech may very well be a desired signal. However, the above described adaptive systems will try to damp this speech signal in order to minimize the output signal, and thereby increase the microphone noise. Furthermore, in quiet situations when the person is not speaking, the adaptive system will try to damp the microphone noise. This results in dynamically undesired damping of the actual desired signal and a significant modulation of the microphone noise, reducing speech perception and sound quality.
A supposed solution to the problem seems to be to modify the microphone signals as input signals for the adaptive function or to modify the output signal as the control signal in the adaptive function. Such modifications have the following drawbacks. One problem is that the possible modifications of signals in the signal path, e.g. filtering away undesired frequency areas, are very limited, because the following adaptation algorithm needs the gain and the delay information of the input signals to be able to adapt correctly. For this reason, a signal modification that e.g. just leaves the envelope of the two microphone signals is not possible.
Another problem is that adaptive systems generally should adapt the output signal relatively soon after the input signals have changed. If not, the system would adjust the characteristic only after a certain delay in which the system is not correctly adapted.
Furthermore, adaptive systems generally should receive the response to a parameter change relatively soon after the parameter has changed. If not, the system would change the parameter further in a certain direction, before getting the response that the parameter change in this direction was in fact erroneously. As a result, such an adaptive system with a delayed response will not reach its optimum very precisely, if at all, and may become unstable.
Algorithms for separating signals with different characteristics, e.g. separating noise from desired speech signals, generally need a certain amount of time to react, e.g. when applying a filter function. Therefore, the implementation of such algorithms in the signal path either prior to the adaptive function or in the feedback path, e.g. by providing a noise pass filter for the output signal controlling the parameter adjustment, conflict with the desire for having a fast response in order to make the adaptive system work, so that such signal modifications do normally not work in most cases.
On this background, it is an object of the present invention to provide an adaptive system and a method of the kind defined, in which the deficiencies of the prior art are remedied and, in particular, to provide a method and an adaptive system of the kind defined which allow to minimize undesired signals without adversely affecting desired signals, even if the desired signals are coming from other directions than the main or frontal direction.